Try the following exercise. In the diagram below, count the number of white squares and count the number of black squares and make a note of your answer. The repeat the exercise with the circles, only this time count how many of the circles would you say are "light" and how many are "dark".

If you got the numbers 5 white squares and 4 black squares, you were right. However it is unlikely that you found it easy to classify the circles as "light" or "dark", and your answer is probably different from the next person who reads this.

All classification schemes start off this way; what is being classified? As the circles and squares show, it is not always easy to unambiguously take any one individual from a mixture and say clearly that it has all the characteristics of its group. White squares are all the same, black squares are all the same, but, lacking a clear separation, how do you unequivocally define a 'light' or a 'dark' circle?

The problem is no different in biology. Gather together members of a species, examine them for one or more characteristic and the results are more likely to be similar to the 'circles' than the 'squares'.

In the middle of the 18th century, however, there was a lot less doubt and a lot more certainty about what constituted a biological species.

Carolus Linnaeus, a Swedish botanist, certainly had no doubts. The bible told him that all living things had been created divinely at the beginning of the world, and had remained that way ever since. His task was first to identify all these god given units, describe their structures, and then use the information he obtained to group them according to the structural patterns he found. There was no ambiguity, species were fixed for all time.

True, new species seemed to arise through selective breeding, but were they really species or just bizarre varieties? There was tantalizing evidence that some species could go extinct and possibly be replaced by others, but the evidence was vague and heavily discounted by systematists of the day. Because of the way they were created, each species had a perfect pattern, a type, or divine plan that was followed when making more individuals.

An archetype was a primal, perfect pattern, an abstraction of a perfect member of the species. In this way annoying individuals with their variations could be comfortably ignored, and classification schemes could then be based on what species should look like, not what they actually looked like.

By the time that Gregor Mendel, Brother Gregory in these pages, came along in the middle of the 19th century, hybrids were a clearly recognized phenomenon in nature and in his own back garden. They could not be ignored. Paleontologists were digging up fossils of long extinct creatures, Darwin and others were putting forward evolutionary ideas and most scientists felt an uncomfortable vagueness about what constituted a species.

Species, if they could be defined as such, didn't seem to have a perfect 'plan' or type, from which they were all made. It was beginning to look as if the species present today were the result of 'hybridizations' and blending of other species in the past. Also, if Mendel's own work was to be believed, present day species could be manipulated to give different forms in the future.

A "transition view" of species arose that was influenced by the work of Darwin and the relationships between species. If species shared a common ancestor, then present anatomical and morphological characters could still be used to classify organisms, but sensitivity had to be shown to the obvious fact that species were mutable (changeable) and that they could adapt to changing circumstances.

Under these circumstances, with species in constant flux, it was necessary to find within the species mixture a single individual that clearly represented all of the 'best' or characteristic features of its kind. Systematists searched for those characters that defined the species unambiguously, and could then be used in classification schemes. The 'Archetype' concept gave way to "perfect" specimen; variation was a nuisance, but you could live with it.

It was uncomfortable, but both the Linnaean view and the transitional view of species had to ignore the variations that were found within natural populations of all species. Darwin had made this variation the cornerstone of his evolutionary mechanism, it was a universally recognized phenomenon, and, ultimately could not be ignored in the process of classification.

The 'Modern View' of a species embraces the population concept. When collecting data, systematists study populations of organisms, not types or individuals. The systematic unit is the interbreeding group of individuals that are clearly related to one another. Characteristics such as morphological details, breeding habits, anatomy, physiology, etc., are the sum of those found in the entire group, not just in one individual.

A "Species" is now defined in terms of populations of individuals sharing a common ancestry that live and breed together in a common habitat. There are rules; (1) individuals must interbreed with one another. In population and evolutionary genetics it is said that they share a common gene pool, and (2) the study group must be reproductively isolated from other groups or species.

Dogs and cats are considered to be different species because they cannot interbreed with one another; they are reproductively isolated. But calicos, tabbies, Persians, and grays, while looking somewhat different to one another are, never the less, still cats and belong to the same species because they can all interbreed.

Membership in a species is no longer defined just in anatomical terms, or by any simple characteristic. Such a membership depends on seeing the individual in its natural setting where its anatomy, its behavior, its interbreeding patterns, its ecology, and its interactions all serve to show how it relates to that population and how similar it is to the other individuals within the group.